The invention relates to transmission of speech, video information, etc. coded with a variable bit rate (VBR) in a packet radio network, such as an ATM network, which uses fixed-length packets.
Use of ATM technology is becoming common in wired packet networks. The asynchronous transfer mode ATM has been developed for use in a broadband ISDN network. In ATM data transfer information is transmitted in packets with a standard length of 53 bytes, called ATM cells. In each cell five bytes consist of the cell header and the rest 48 bytes are payload, i.e. real data. ATM cells have been specified in ITU-T recommendations I.361 and I.150. To put it simply, the user information to be transmitted is split into fixed-length bit strings and each bit string is inserted into the information field of the ATM cell. The number of bit strings per time unit represents the transmission capacity required by the user. A header, which will be described in greater detail below, is also added to the information field, the result being an ATM cell with a standard length of 53 bytes. The cell is an independent data transmission unit because it indirectly includes information on the receiver""s address on the basis of which the receiver is found in the network. The ATM network comprises various pre-defined service criteria for which a large number of different parameters can be set. These criteria include response time, bit error rate and probability of packet loss.
The ATM is a connection-oriented packet network, which means that connections are established and terminated according to standardized practices. A connection between two parties through the ATM network is called an ATM virtual channel. The advantages of the ATM include the fact that different services can be offered flexibly. For example, all bandwidths are equally possible within the capacity range of the physical layer in the ATM network (from 1.5 to 622 Mbps with existing technology).
ATM technology can also be used for transmitting coded speech and/or video signals, etc. If necessary, such a speech and/or video signal will be referred to as an AV signal below, but transmission of a speech signal in the ATM network will be used as the practical example. A general problem related to this case is that efficient coding of speech (in respect of the bandwidth) produces a signal with a variable bit rate (VBR).
If samples are taken from a speech signal at a fixed resolution and rate, the information content of the resulting digital signal per time unit is constant. However, since it is always practical to pack, i.e. compress, a speech-like signal before transmission, the compressed signal has a variable bit rate.
FIG. 1 is a block diagram of an apparatus in which the invention can be applied. Sample packets S (e.g. speech frames) are formed from an input signal 1 in a coder 2 by subjecting the input signal to A/D conversion and compression. A sample period P with a fixed duration is related to each sample packet. An ATM adapter 3 inserts the sample packets S into the payload part PL of the ATM cells 4. The recipient""s identifier is inserted into the header H of the ATM cell. ATM cells are transmitted to a receiver via an ATM network 5, and the ATM adapter 6 of the receiver separates sample packets from the payload parts of the packets 4 and supplies them to a decoder 7 for producing an output signal 8.
FIG. 2 illustrates conventional technology for transmitting VBR coded speech packets in the ATM network. Curve BR (Bit Rate) shows speech information content per time unit (e.g. kbit/s). Samples are taken from the input signal 1 and coded into speech or sample packets S1, S2, etc. The respective sample periods are P0, P1, etc. Each sample packet Si can naturally be transmitted only after the sample period Pi in question has ended, at moment Ti+1. In FIG. 2 the width of each sample packet Si illustrates its information content. A/D and D/A converters in blocks 2 and 7 are not always necessary, because transcoding, i.e. conversion, between two different coding methods is also possible. It should also be noted that the curve BR is only used to facilitate understanding of the invention. The coder does not have such information available, at least not in advance, but it produces corresponding information when coding the input signal.
The problem related to the technology of FIG. 2 described above is that bandwidth utilization is poor. This can be seen particularly in sample periods P3 to P5 when there is a pause in speech and silence descriptors SID, which the receiver replaces with comfort noise, are typically transmitted in mobile communication systems. A solution to this problem is disclosed in published commonly assigned PCT application WO97/48211. According to the art disclosed in it, ATM cells are filled with speech frames until the ATM cell is substantially filled up. However, correspondence between the ATM cell and speech frame is lost because there are typically whole speech frames in the middle of the ATM cell, whereas at its beginning and end there are parts of speech frames. This may cause problems e.g. during handover because in downlink traffic the first part and the last part of the ATM cell should be transmitted via different base stations.
An object of the invention is to provide a method and an apparatus implementing the method to eliminate the above-mentioned problems. The objects of the invention are achieved with a method and system which are characterized by what is disclosed in the independent claims. Preferred embodiments of the invention are described in the dependent claims.
The invention is based on the idea that the size (information content) of the sample packets should be kept substantially constant. This is achieved by changing a coding parameter, such as duration of the sample period P or the coding method used. The coding method can be changed simply by arranging the coder 2 to form different sample packets S, Sxe2x80x2 and Sxe2x80x3 from the same sample period, and by selecting the largest sample packet that fits in the payload part PL. When the information content per time unit BR is low, a longer sample period is used and/or better coding is produced, and vice versa. However, this seemingly simple principle causes some new problems to which solutions are disclosed in connection with the preferred embodiments of the invention.
An advantage of the method and system of the invention is that the bandwidth is utilized optimally while one-to-one correspondence can be maintained between the sample packet and the ATM cell. When one sample packet corresponds to one ATM cell, error correction due to cell loss is a relatively clear procedure because only one sample packet is lost.